First off, no question is noobish. You just need a little help in understanding the game, is all
Cargo is the term used by many pirates for the plunder you get when you sink ships (i.e. treasure chests, cargo crates, etc.) Cargo can sometimes also be a direct term for Cagro crates, which is a type of cargo itself.
As said, cargo can be obtained by sinking ships out on the open sea. There are 3 different types of cargo: Cargo crates, which hold the least valuable plunder, treasure chests, with usually nice plunder, and Royal Chests, which usually hold the most valuable plunder. These types of cargo are usually spread across by level, where the higher leveled the ship, the more likely you will recieve better cargo (although sometimes not the case.) When you port, the cargo then amounts to gold, with each different item that is held within the cargo equaling a different amount of gold.
Hope that helped
Pirate on!